Fermentation Design 6.0 THE DESIGN OF LARGE FERMENTERS (BASED ON AERATION 6. 1 Agitator effectiveness Laboratory scale work frequently reports aeration rates as the volume of air at standard conditions per volume of liquid per minute, or standard cubic feet of air per hour per gallon. Production engineers realized that the scale-up of aeration for a large range of vessel sizes was by superficial linear elocity (SLV), or feet per second. Large scale fermenters, for energy savings in production equipment, use air-agitated fermenters. The cost savings are not apparent when comparing the cost of operating a fermenter agitator to the cost of the increased air pressure required. However, when the total capital and operating costs of fermentation plants (utilities included) for the two methods of fermentation are compared, the non-mechanically agitated fer- menter design is cheaper. The questions are, How much mixing horsepower is available from aeration, versus how much turbine horsepower is effective r aeration and mixing? D N. millerlll of DuPont, describing his results of scale-up of an agitated fermenter states, both Ki a and gas hold-up increase with an increasing gas rate and agitator speed. Gas sparging is the stronger'effect and tends to be increasingly dominant as gas rate increases At superficial gas velocities, 0.49 ft/sec and higher, very little additional mass transfer improvement can be gained with increased mechanical energy input. "Otto Nagel and associates 2) found in gas-liquid reactors that the mass transfer area of the gas in the liquid is proportional to the 0. 4 power in the energy dissipation. Thus for a 50 hp agitator, 12 hp directly affects the mass transfer area of oxygen. The upper impellers mainly circulate the fluid and contribute very little to bubble dispersion and oxygen transfer. Most of the agitators power is spent mixing the fluid. Tounderstand mixing theories see Brodkey, Danckwerts, Oldshue or other texts. 3)The primary function of mixing for aerobic fermentations is to increase the surface area of ai bubbles(the interfacial surface area)to minimize the bubble diameter. The fermenter is not the same as a chemical reactor where first and second order reactions occur between soluble reactants. The dissolution rate of oxygen into fermentation broth is controlled by diffusion. The consumption of soluble oxygen by the organism is an irreversible reaction and unless sufficient oxygen diffuses across the air- liquid surface area, the fermentation will cease aerobic metabolism. Methods offorcing more air into solution are more interfacial surface area, more air/oxygen, higher air pressure, reduced cell volume, or controlling metabolism by reduced carbohydrate feed ratesFermentation Design 99 6.0 THE DESIGN OF LARGE FERMENTERS (BASED ON AERATION) 6.1 Agitator Effectiveness Laboratory scale work frequently reports aeration rates as the volume of air at standard conditions per volume of liquid per minute, or standard cubic feet of air per hour per gallon. Production engineers realized that the scale-up of aeration for a large range of vessel sizes was by superficial linear velocity (SLV), or feet per second. Large scale fermenters, for energy savings in production equipment, use air-agitated fermenters. The cost savings are not apparent when comparing the cost of operating a fermenter agitator to the cost of the increased air pressure required. However, when the total capital and operating costs of fermentation plants (utilities included) for the two methods of fermentation are compared, the non-mechanically agitated fer￾menter design is cheaper. The questions are, How much mixing horsepower is available from aeration, versus how much turbine horsepower is effective for aeration and mixing? D. N. Miller[”] of DuPont, describing his results of scale-up of an agitated fermenter states, “both &a and gas hold-up increase with an increasing gas rate and agitator speed. Gas sparging is the ‘stronger’ effect and tends to be increasingly dominant as gas rate increases. At superficial gas velocities, 0.49 Wsec and higher, very little additional mass transfer improvement can be gained with increased mechanical energy input.” Otto Nagel and associates[12] found in gas-liquid reactors that the mass transfer area of the gas in the liquid is proportional to the 0.4 power in the energy dissipation. Thus for a 50 hp agitator, 12 hp directly affects the mass transfer area of oxygen. The upper impellers mainly circulate the fluid and contribute very little to bubble dispersion and oxygen transfer. Most of the agitator’s power is spent mixing the fluid. (To understand mixing theories see Brodkey, Danckwerts, Oldshue or other texts.[I3]) The primary function of mixing for aerobic fermentations is to increase the surface area of air bubbles (the interfacial surface area) to minimize the bubble diameter. The fermenter is not the same as a chemical reactor where first and second order reactions occur between soluble reactants. The dissolution rate of oxygen into fermentation broth is controlled by diffusion. The consumption of soluble oxygen by the organism is an irreversible reaction and unless sufficient oxygen diffuses across the air-liquid surface area, the fermentation will cease aerobic metabolism. Methods of forcing more air into solution are: more interfacial surface area, more aidoxygen, higher air pressure, reduced cell volume, or controlling metabolism by reduced carbohydrate feed rates